August 29, 2009

Kinesin needs a MAP to step in time

The complex choreography
of chromosome segregation in anaphase is split into two processes: Anaphase A
in which sister chromatids move to opposite spindle poles and Anaphase B in
which the spindle elongates.The latter
occurs when overlapping, antiparallel microtubules that constitute the ‘central
spindle’ or spindle midzone slide past each other.The microtubule-associated protein (MAP)
PRC1/Ase1 is required for midzone assembly and stability of the anaphase
spindle (Khmelinskii et al., 2007;
Schuyler et al., 2003).In work
published in The JCB, Scheibel and
colleagues (Khmelinskii et al., 2007) showed that in budding yeast Ase1
dephosphorylation by the phosphatase Cdc14 is important for midzone formation
and spindle elongation.

Image from Khmelinskii et al. (2007).Ase1 (green)localizes to the spindle midzone
in anaphase

In the August
issue of Developmental Cell,
Khmelinskii et al. (2009) present the next step in this story.They show that Ase1 dephosphorylation
promotes its interaction with the kinesin Cin8 thereby recruiting Cin8 to the
central spindle where it drives spindle elongation.This function of Ase1 appears to be conserved
in fission yeast as shown by Fu et al. (2009) in the same issue. These studies,
together with previous work, show how the timing of Anaphase A and B are
coordinated.In early mitosis, Ase1 is
phosphorylated by Cdk1 thereby preventing spindle elongation.At the metaphase to anaphase transition, separase—which
cleaves cohesin to promote sister chromatid separation—activates Cdc14 through
the FEAR network.Cdc14 dephosphorylates
Ase1 which moves to the spindle midzone and recruits, in addition to many other
proteins, Cin8, which slides antiparallel microtubules apart to promote spindle
elongation.

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